1. Related Field
The present invention relates to the field of recording digital media such as video or audio, where compression is used to reduce the amount of data to save onto a data storage. In particular the invention relates to recording digital media, wherein the memory area required to store the digital media is unknown. This could e.g. be a media stream comprising an unknown number of media frames or a media stream with unknown content. The invention also relates to a corresponding media recorder and computer program product for recording streamed digital media.
2. Description of Related Art
When recording digital streamed media the recording time and thereby the size of the recorded media, is often not known beforehand. One example is a helicopter, having a video or infrared camera, recording digital media during a mission. In such an application the flight time may be unknown. Therefore, it is not possible to beforehand determine the size of the memory or other data storage required to store the recorded data.
In a state of the art real time video compression system, one cannot go back and recompress a frame with a different target rate afterwards. Each frame passes the videocompressor only one time and the compression rate needs to be specified beforehand. Therefore, one often has to compromise between bitrate, i.e. quality, and the risk of memory overflow. There is also a risk that an unnecessarily large memory is chosen, in order to avoid the risk of memory overflow. This is a problem, because a data storage such as a memory is often a costly component in digital video recorders. Therefore, it is not desired to have larger memory capacity, i.e. make it more expensive, than it has to.
In order to avoid memory overflow, it is common to have a required minimum recording time in digital video recorders. The traditional way to fulfil this requirement is by using a video compression algorithm in a Constant Bitrate (CB) mode. This entails that the maximum bitrate is known, and therefore it is possible to dimension the data storage to ensure that it's capacity is big enough for the required minimum time. The required minimum recording time, normally comes from the defined maximum mission time for the actual craft/crew. In reality, the majority of the missions will however not be near as long as the defined maximum mission time. This means that for the majority of the missions only a part of the available memory capacity will be used, which implies that video could have been stored with higher bitrate i.e. better quality, than it was. The difficulty is to know the mission length before the mission is started, which is impossible.
In some applications there may also be several media inputs, sharing one memory. In such a case, the available memory is dependent on the number of active sensors during flight time, which may also not be known beforehand. A helicopter may comprise one video camera and one IR camera. In many occasions only one camera is used. However, during flight the crew may choose to activate a second camera. As this will not be known beforehand, an unnecessarily low bitrate will be used in order to leave space for a possible second media input. However, this implies that in many applications an unnecessarily low bitrate is used for recording, even when there is a lot of free memory space when the recording is finalised.
Similar problems may occur in any application comprising a media recorder wherein the recording length or the size of the encoded data is not known beforehand, e.g. in a video camera an audio recorder, a mobile phone or in a surveillance camera.
Considering the mentioned drawbacks with current technology, finding a way to optimize the usage of the memory in a digital media recorder, would be most welcome.